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Concept
Definition of planet
Summary
The definition of planet has changed several times since the word was coined by the ancient Greeks. Greek astronomers employed the term ἀστέρες πλανῆται (), 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different celestial bodies, from the Sun and the Moon to satellites and asteroids.
In modern astronomy, there are two primary conceptions of a 'planet'. Disregarding the often inconsistent technical details, they are whether an astronomical body dynamically dominates its region (that is, whether it controls the fate of other smaller bodies in its vicinity) or whether it is in hydrostatic equilibrium (that is, whether it looks round). These may be characterized as the dynamical dominance definition and the geophysical definition.
The issue of a clear definition for planet came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto. In its August 2006 response, the International Astronomical Union (IAU), recognised by astronomers as the world body responsible for resolving issues of nomenclature, released its decision on the matter during a meeting in Prague. This definition, which applies only to the Solar System (though exoplanets had been addressed in 2003), states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects approaching its orbit. Pluto fulfills the first two of these criteria, but not the third, and therefore does not qualify as a planet under this formalized definition. The IAU's decision has not resolved all controversies, and while many astronomers have accepted it, some planetary scientists have rejected it outright, proposing a geophysical or similar definition instead.
Geocentric modelHeliocentrismCelestial spheres and Classical planet
While knowledge of the planets predates history and is common to most civilizations, the word planet dates back to ancient Greece.
Official source
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